Ok ... looking at your image (and this is off the top of my head so be warned), get rid of the cap between pins 2 & 10, remove connection between 3 & 9 (just leave them dead pins unless you have a reason to short them), remove resistor on powerline and run 1 12v line to pins 7 & 8 they can share the power line, if you are controlling this with arduino set pin 5 to HIGH and 6 to LOW and reverse that for spinning the other direction. I don't? have any resistors or caps in my setup with lb1641 ... and get rid of the diode? on pin 4.

Ha, works. It wasn't the diode, it wasn't the capacitor, it was my power supply. I used an old printer power supply that outputs 32V, and a LM317 that gets that down to 12V.I swapped that for a 12V industrial power supply and now everything works and the world makes sense again. The Capacitor reduces noise, and the diode controls how much current flows through the IC, just like the manual says.Your diagram with the 9V battery made me think.... It was the only thing that was different about your setup. Apart from that, everything was identical and mine didn't work.